(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same. More particularly, the present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same in which a structure of the positive active material is made more stable.
(b) Description of the Related Art
There is an ever-increasing demand for rechargeable lithium batteries in electronic devices such as cellular phones, camcorders, and laptop computers. The positive active material in the rechargeable lithium battery is the main factor in determining the capacity of the battery. The electrochemical characteristics of the positive active materials influence the high-rate cycle characteristics and the capacity retention of the battery during service life.
Cobalt-based active materials have a higher capacity than manganese-based active materials, and have a higher degree of structural stability than nickel-based active materials. Thus, among all of positive active materials used in rechargeable lithium batteries, the cobalt-based active materials such as LiCoO2 are the most commonly used (over 95% of rechargeable Li-ion batteries in the market today use LiCoO2). In recent times, there has been ongoing research on enhancing charge capacity by increasing the charge voltage range.
Li1xe2x88x92xCoO2 has the most stable structure of all the positive active materials used in rechargeable lithium batteries. Although the compound has a stable structure of xcex1-NaFeO2, the x value affects the structural stability. With rechargeable lithium batteries comprising LixCoO2 within the limits of 0 less than x less than 0.5 as the active material, the structure of the LixCoO2 is stably maintained on charging and discharging cycling. However, when the x value is 0.5 or greater, the structure becomes unstable as a result of a phase transition causing abrupt decreases of capacity. This phase transition is attributed to a monoclinic phase formed at an x value approaching 0.5. The formation of the monoclinic phase causes a sharp reduction in a c-axis in the crystalline structure. When the x value is below 0.5, reduction of the c-axis occurs with formation of a hexagonal phase. When the x value is in the range between 0 and 0.7, the c-axis reduction is as much as 1.7%. On the other hand, the a-axis undergoes almost no change regardless of the x value.
Such non-uniform dimension changes stress particles unevenly, and at an elasticity limit of greater than 0.1%, fractures develop in the particles. The generation of such mechanical fractures directly results in a reduction in capacity. That is, maximally repressing the generation of a monoclinic phase of LiCoO2 is a way to obtain structural (cyclic) stability. To this end, U.S. Pat. No. 5,705,291 of Amatucci et al. discloses a method in which a surface of the cobalt-based material is coated with a composition comprising a borate, aluminate, silicate, or mixtures thereof. However, a poor structural stability results with the application of this method.
It is an object of the present invention to provide a positive active material for a rechargeable lithium battery having a stable structure.
It is another object of the present invention to provide a method of preparing a positive active material for a rechargeable lithium battery having a stable structure.
These and other objects may be achieved by a positive active material for a rechargeable lithium battery comprising a LiCoO2 core and a metal selected from a group consisting of Al, Mg, Sn, Ca, Ti, Mn and mixtures thereof. The metal has a concentration gradient from the surface of the core to the center of the core. That is, the surface concentration of the metal is higher than the core concentration thereof.
To achieve these objects, the present invention provides a method of preparing the positive active material for a rechargeable lithium battery. In this method, a metal compound is dissolved in alcohol to prepare a metal compound solution in a sol state, the metal compound solution in the sol state is coated on LiCoO2, and the coated LiCoO2 is sintered at 150 to 500xc2x0 C.